Various electromagnetic techniques exist to perform surveys of a subterranean structure for identifying elements of interest. Examples of elements of interest in the subterranean structure include subsurface resistive bodies, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, and fresh-water aquifers. One survey technique is the magnetotelluric (MT) survey technique that employs time measurements of electric and magnetic fields (which are responsive to naturally occurring electromagnetic fields) for determining the electrical conductivity distribution beneath the surface. Another survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic transmitter, called a “source,” is used to generate electromagnetic signals. With either survey technique, surveying units, called “receivers,” are deployed on a surface (such as at the sea floor or on land) within an area of interest to make measurements from which information about the subterranean structure can be derived. The receivers may include a number of sensors for detecting any combination of electric fields, electric currents, and magnetic fields.
In marine environment CSEM surveys, modeling and acquisition studies have shown that thin resistive targets in a subterranean structure, such as hydrocarbon-bearing reservoirs, gas injection zones, thin carbonate or salt layers, fresh water aquifers, and so forth, are more easily detectable when a CSEM source is positioned close to the sea floor. In practice, the CSEM source is towed, or “flown,” as close to the sea floor as conditions will allow. Typically, the CSEM source will be towed between 30 to 50 meters above the sea floor.
Usually, when performing CSEM surveying, EM receivers are placed on the sea floor. An issue associated with deploying EM receivers on the sea floor is that such deployment is both labor and time-intensive. Also, after the surveying is completed, retrieving or recovering the EM receivers from the sea floor is also a labor and time-intensive process. Moreover, sea floor receivers tend to measure a total EM field that contains the response of not only targets of interest, but also the response of sea water, and in a shallow water environment, the response of air above the sea water.